Ecogeek quotes a European study
“As a general rule, additional wind replaces coal during hours of low power demand and gas during hours of high demand, but ‘the overall amount the price goes down depends on the power mix in the country, and how much more expensive the fuels are that wind is replacing.'”
While the study was commissioned by a wind energy association, the results do seem plausible and they note that wind power in Europe is overtaking gas for power production.